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Typhoon Vamei: Born At The Equator
A long-standing weather rule held that tropical cyclones never form within 180 miles of the Equator. In late-December 2001, however, a birth changed all that.
Tropical meteorologists long believed the belt 10 degrees either side of the Equator was cyclone-free because the Coriolis effect was too weak to spin a budding tropical depression enough to form or sustain an organized storm rotation.
The Coriolis effect, produced by the Earth's rotation, is non-existent directly on the Equator and increases in magnitude as one travels toward the Poles. It's the force that gives mid-latitude cyclones their spin and veers all large-scale motion toward the right in the Northern Hemisphere. It does not affect smaller scale flows such as dust devils, nor bathtub drains. (Sorry to stomp on that myth!)
When pockets of intense tropical thunderstorms form over warm equatorial ocean waters, they don't develop the characteristic counterclockwise spin of tropical storms unless they move to higher latitudes. Meteorologists believed the Coriolis force was needed to get the system spinning. The spin, when properly fuelled, allowed a storm to gain intensity.
But, December 27, 2001, in the South China Sea, 1.5 degrees, about 100 miles, north of the Equator tropical forecasters announced the appearance of Typhoon Vamei, and long-held beliefs changed.
Analysis later revealed that a weak, quasi-stagnant disturbance off Borneo interacted with a strong, cold surge off Asia that set up a background rotation when it hit the island. When surge met disturbance, spin happened, and a typhoon rapidly emerged that had winds howling in both hemispheres.
Vamei's 87-mph winds damaged the carrier USS Carl Vinson and an accompanying ship.
Prior to Vamei, Typhoon Sarah in 1956 formed closest to the Equator, 3.3 degrees north. Such confluence of events has been estimated to occur once every 100 to 400 years.
[Vamei satellite photo courtesty US Naval Pacific Meteorology and Oceanography Center.]
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